The long-term goal of this project is to understand the molecular mechanisms of pathogenesis of influenza viruses. Influenza A viruses continue to cause significant disease in humans as well as many other animal species. Influenza pandemics have occurred for centuries and another pandemic that affects 20-40 million US citizens and results in tens of thousands of deaths is very probable, if not certain. Thus, it is important that we understand the processes critical to pathogenesis in order to prepare for future pandemics. Some of the mechanisms that lead to increased pathogenesis have been elegantly demonstrated (e.g., hemagglutinin cleavage). However, many molecular mechanisms important in viral pathogenesis and interspecies transmission remain elusive. Avian H5 viruses, such as the viruses responsible for 18 zoonotic infections in the "bird flu" outbreak in Hong Kong in 1997, provide a powerful model of pathogenesis because they range from avirulent to highly virulent. Furthermore, H5 influenza viruses with zoonotic potential continue to circulate and cause disease in multiple species in North America, Europe and Asia. Preliminary studies have identified a putative internal promoter in the genome of a pathogenic strain of H5 influenza. This proposal will (1) determine if a novel transcription promoter in H5 viruses results in the production of subgenomic mRNA that yields a unique protein, (2) identify a correlation between virulence and use of the putative promoter; and (3) investigate the specific nucleic acids and proteins important in the utilization of this RNA element. Addressing these specific aims will expand our understanding of influenza promoter utilization and identify processes important in pathogenesis and the emergence of pandemic and epizootic strains of influenza virus.